Quasi four-level solid-state lasers operating in the 2 .mu.m wavelength region are used for sensing wind velocity, as optical pumps for mid-infrared parametric oscillators, for remote sensing of water vapor or carbon dioxide, for medical applications such as laser angioplasty, for material processing applications, and for communications.
In general, quasi four-level lasers operating in the 2 .mu.m wavelength region utilize laser material made from a host material doped with laser active ions from the group of rare earth ions such as holmium (Ho) and thulium (Tm), transition metal ions such as chromium (Cr) or combinations thereof. The host materials for such lasers come from the group of crystals such as YAG, YLF, YSGG, GSGG, GSAG, YSAG, YAlO, GGG, YGG and LLGG. To date, the Tm:YAG and Tm:YLF lasers have provided the best performance for diode-pumped, quasi four-level lasers operating at room temperature. However, the thresholds of these lasers are relatively high which can lead to thermal stresses in the laser crystals or rods as higher input energies are required.